In recent years, wandering of elderly persons suffering from dementia and accidents involving stumbling and falling in the vicinity of the bed have become a social problem, and a solution to this problem is needed. There are high expectations that a quicker response can be achieved during an abnormal situation by monitoring the on-bed movements of elderly person, patients that have recently undergone surgery, or other users who are bedridden over a long period of time, by using the load information of the bed.
The art disclosed in patents document 1 merely uses weight information to determine whether a user is still on the bed or has left the bed, and cannot monitor the movement of the user on the bed.
The arts disclosed in patent documents 2 to 4 and non-patent document 1 solve this problem by calculating the center of gravity from the weight information of four points on the bed, and use the center-of-gravity position information to monitor the movement of a user in bed.    Patent Document 1: Japanese Laid-Open Patent Application No. 2-280733    Patent Document 2: Japanese Patent No. 3093745    Patent Document 3: Japanese Patent No. 3322632    Patent Document 4: U.S. Pat. No. 5,276,432    Non-patent Document 1: J. C. Barbenel et al., Monitoring the mobility of patients in bed, Medical & Biological Engineering & Computing, pp. 466-468 (September 1985)